冷卻速率與鑭添加對Sn-3.5Ag無鉛銲料銲點微結構性質與機械強度之影響

Abstract

The effect of Ag3Sn morphologies, Ag3Sn coarsening and excessive growth of IMC layer are concerned with the problems of Sn-Ag solder. The aim of this study is expected to solve the problems of Sn-Ag solder. In evolution of Ag3Sn morphology, an experimental investigation is performed to examine the effect of the cooling rate on the morphology of the Ag3Sn intermetallic compounds (IMCs) formed during the solidification of bulk eutectic Sn-3.5Ag solder and Sn-3.5Ag/Cu joints. It is shown that the cooling rate has a significant effect on the solidification time and therefore influences both the size and the morphology of the final Ag3Sn compounds. Specifically, the Ag3Sn compounds exhibit a particle-like → needle-like → needle-like with plate-like tails → plate-like → large plate-like evolution as the cooling rate is reduced. The large plate-like Ag3Sn compounds are observed only in the Sn-3.5Ag/Cu specimens. The large plate-like Ag3Sn formed at the interface layer due to the formation of a Cu6Sn5 IMC layer at the interface. Thus, it is inferred that the Cu6Sn5 interfacial layer in the Sn-3.5Ag/Cu specimens leads to a local enrichment of Ag at the interface and prompts the formation of large Ag3Sn IMCs via the Cu6Sn5 heterogeneous nucleation sites. The tensile strength and ductility of Sn-3.5Ag solder drops apparently with Ag3Sn evolution by cooling rate decreasing due to the fracture behaviour changes. The crack easily initiates and propagates in the solder when Ag3Sn compounds grow into plate-like thus causing stress concentration effect at β-Sn/Ag3Sn plate interface. Therefore, Sn-3.5Ag containing Ag3Sn plates have lowest mechanical properties. The effects of lanthanum (La) additions on the microstructure, microhardness, adhesive strength and fracture behavior of Sn-3.5Ag solders. Sn-3.5Ag-xLa ternary solders were prepared by adding 0-1.0wt% La to Sn-3.5Ag alloy. The Sn-3.5Ag-xLa solders comprised -Sn, Ag3Sn, and LaSn3 phases, and their microstructure was refined by La additions. The electron backscatter diffraction (EBSD) results confirmed that the face-centered cubic (fcc) structure of LaSn3 compounds distributed randomly in the -Sn matrix had no obvious preferred orientation. As-cast, the addition of La increased the microhardness of the Sn-Ag solder due to the refining effect of Ag3Sn particles and increased formation of LaSn3 compounds. As aging time was increased, the microhardness of the solders decreased and the Ag3Sn compounds coarsened. However, the coarsening of Ag3Sn compounds was retarded by the La, and the size and amount of LaSn3 compounds did not change perceptibly with aging time. La additions reduced the ripening reaction rate of the scallop-type Cu6Sn5 compounds at the interface layer. IMC layer in the aged samples, trace amounts of La addition suppressed the Sn diffusion and retarded the growth of the Cu6Sn5 layer, but not that of the Cu3Sn layer. In spite of the significant refinement of microstructure and suppression of the growth of Cu6Sn5 IMC layer caused by the La additions, the expected enhancement of adhesive strength and ductility did not occur apparently. With the aid of Argon Beam Milling, we have found defects and voids inside the LaSn3 compounds; these features were closely related to the crack initiation and formation during the adhesive tensile test. Consequently, the beneficial effect due to the refinement of microstructure and suppression of interfacial layer compounds was retarded.